GENEVA, June 7 (Reuters) - The United Nations expects 10.25 million Syrians, half the population, will need humanitarian aid by the end of 2013 at a cost of more than $5 billion, U.N. humanitarian agencies said on Friday.

The new forecasts, in an updated Syria response plan, include more than a doubling of the refugee population to 3.45 million from 1.6 million now, spread across Lebanon, Jordan, Turkey, Iraq and Egypt.

But it forecasts no increase in the number of Syrians within the country who will need aid between now and the end of the year, with the figure put at 6.8 million.

The U.N. World Food Programme (WFP), which has delivered 500 million meals in Syria so far this year, expects its weekly costs to rise from almost $20 million now to $36 million after September. It says it has a funding shortfall of about $725 million.

"These are huge numbers. They're not sustainable over the very long term," said WFP Deputy Executive Director Amir Abdulla.

The WFP's Syria Regional Emergency Coordinator Muhannad Hadi said: "We have reached a stage in Syria where some of the people, if they don't get food from the World Food Programme, they simply do not eat."